The existence of the thioredoxin (hereinafter abbreviated as “TRX” in the specification)/thioredoxin reductase system is known as one of the reduction-oxidation pathway of thiol group. The system regulates reversible reduction-oxidation reaction of thiol group and maintains a constant thiol level in vivo so as to prevent functional depression of thiol protein by formation of disulfide bonds and advancement of peroxidation state.
It has been elucidated that thioredoxin reductase has activity of reductively cleaving a disulfide bond of a target protein in the presence of NADPH and thioredoxin, as well as a variety of other physiological activities. Thioredoxin, a substrate for thioredoxin reductase, is a protein containing two thiol groups in the molecule, and functions also as a hydrogen donor in reduction of ribonucleotide by ribonucleotide reductase.